Honeywell wifi thermostat needs 24volts but boiler transformer only 16volts. Need wiring diagram adding 24 volt transfomer to power thermostat

Without seeing the wiring diagram, I can only say that the MV/PV terminal in many cases is used as the common from the thermostat/transformer. Usually, the 24 volts is supplied to the thermostat from a step down transformer and the thermostat sends voltage to the controller. I would hesitate to "guess" at this without a complete understanding of the wiring requirements. Any HVAC tech should be able to help you.

If you accidentally touched the red wire to any other wires when you were changing the T/stat it's possible you blew a fuse for the 24 volt transformer on the outside unit. If you cant find a fuse, then the transformer needs to be replaced.

The color code for thermostat wiring is as follows.
The "R" terminal is power into the stat from the boiler control transformer. Normally 24 volts. The "Y" terminal is for cooling. Normally left blank for a boiler. The "W" terminal is for heat. This one used to start the heating cycle.The "G" terminal is for fan operation. Probably not used for a boiler. Check the boiler terminal board for these functions and write down the wire color on each one. Match the boiler colors to the right function on the stat. If you need more than this, let me know.

If the boiler has a 24 volt thermostat system, it may not be the thermostat that has failed. It may be the 24 volt inline transformer that's failed. To find out, you will need a circuit tester to make sure power is getting from the transformer to the thermostat. This way, you will know what needs to be replaced.

What are you trying to wire up?
Typical Thermostat Wiring for 4 or 5 wire Heat/Cool Thermostat (not heat pumps)
4 Wire Termostat
R = Red One Side of 24 volt Transformer
G = Green Blower
Y = Yellow Cool
W = White Heat
5 Wire Thermostat
with one transformer for both units
Rc One Side of 24 volt Transformer
Rh Jumper Rc to Rh
G = Green Blower
Y = Yellow Cool
W = White Heat
5 Wire Thermostat
With 2 transformers one for indoor unit
And one for outdoor unit
Rc = Red One Side of (outside) 24 volt (outside transformer)
G = Green Blower (outside transformer)
Y = Yellow Cool (outside transformer)
Rh = Blue One side of (inside) 24 volt (inside transformer)
W = White Heat (inside transformer)
COOLING ONLY Thermostat
R = Red One Side of 24 volt Transformer
G = Green Blower
Y = Yellow Cool
HEATING ONLY
R or Rh One Side of 24 volt Transformer
W = White Heat

what kind of thermostat to you have, did you purchase the thermostat with the unit are you upgrading to a electronic thermostat, If you are using the mechanical thermostat that comes with the unit you will not need a thermostat. please see the link below and page 11
http://www.wfc-fc.com/pdf/Monterey_Top_Vent_IOM.pdf

should have 24 volts @ t-stat. if tis is a boiler system it could have air in the zone that stays cold. bleed the radiator out and this should get the heat into the area you want.

If the thermostat is for just heat and is not a power robbing unit (used 24 volts to power a back lit display) all you need to do is hook one wire to R and one wire to W. If it is a power robbing unit you will need another wire that will connect to either C or B on the thermostat and to a transformer. If you can't run the extra wire from a transformer (supplying power) swap the thermostat to a unit that used a battery for the display.

Open the air handler. Locate the low voltage transformer. One side is 220 volts input. The other is 24 volts output. Take your VOM and touch the test leads to the input to verify 220v in. Then touch the other 2 leads to verify 24v out.
If you have power in and no power out, replace the lv transformer.

What kind of boiler are using is this boiler a on the demand electric or gas? You need low voltage wires. If you get me make and model # of off boiler i can try and find wiring diagram . Tell me a little more about you heat system to. Like baseboard heat or in floor how many zones